Use single underscore for all parameters name and local variables in
bintime_* related functions. This commit completes what was already done
by theraven@ for bintime_shift, and just uses a single underscore instead
of two (which is a style bug according to Bruce). See r251855 for reference.

Reported by:	theraven
Discussed with:	bde
Reviewed by:	bde
Forced dismounts of NFS mounts can fail when thread(s) are stuck
waiting for an RPC reply from the server while holding the mount
point busy (mnt_lockref incremented). This happens because dounmount()
msleep()s waiting for mnt_lockref to become 0, before calling
VFS_UNMOUNT(). This patch adds a new VFS operation called VFS_PURGE(),
which the NFS client implements as purging RPCs in progress. Making
this call before checking mnt_lockref fixes the problem, by ensuring
that the VOP_xxx() calls will fail and unbusy the mount point.

Reported by:	sbruno
Reviewed by:	kib
MFC after:	2 weeks